Scientist has ecstasy in beer mug (8)
I believe the answer is:
einstein
'scientist' is the definition.
(Einstein is an example)
'ecstasy in beer mug' is the wordplay.
'ecstasy' becomes 'E' (informal term for the drug).
'beer mug' becomes 'stein' (synonyms).
'e'+'in'+'stein'='EINSTEIN'
'has' is the link.
(Other definitions for einstein that I've seen before include "Albert, who founded the theory of relativity" , "Albert . . . . . . . ., great scientist from nineties" , "He contributed the theory of relativity" , "renowned scientist" , "German physicist of relativity" .)